It is desirable that an internal combustion engine be operated in a rich region in which an excess air rate .lambda. is .lambda.&lt;1, at .lambda.=1 (a theoretical A/F) and in a lean region in which .lambda.&lt;1 corresponding to the conditions of the engine; hence it is required that the A/F be detected in a wide range from the rich region to the lean region by a single sensor.
On the other hand, relations between the excess air rate .lambda. and a residual oxygen consistency and a carbon monoxide consistency in exhaust gas are as shown in FIG. 11 and in the lean region the oxygen (O.sub.2 ) consistency varies approximately linearly to the A/F and in the rich region the carbon monoxide (CO) consistency varies approximately linearly to the A/F.
Basic principles of A/F sensors of the prior art which detect the A/F of each region individually by utilizing the residual oxygen consistency and the carbon monoxide consistency are shown in FIG. 12 (A)-(C). The A/F sensor is constituted by an electrode 1, a zirconia solid electrolyte 2, an electrode 3, a protecting film 4 and an ammeter 5.
The sensor shown in FIG. 12 (A) detects the rich region (.lambda.&lt;1) by applying an exciting voltage E of approximately 0.5 V between the electrode 1 and the electrode 3 which are a cathode and an anode respectively as known from, for instance, Japanese Patent Laid-Open No. 66292/1978. The protecting film 4 functions as a gas diffusion resistant body and the oxygen gas which is subjected to burning reaction with unburnt gas which is diffused into the electrode 3 part through the protecting film 4 is transferred from the electrode 1 part contacting the atmosphere to the electrode 3 part through the zirconia solid electrolyte 2 in the form of oxygen ions. Therefore, a pumping current Ip measured by the ammeter 5 represents the quantity of the oxygen ions transferred from the electrode 1 to the electrode 3 and corresponds to the quantity of the unburnt gas diffused into the electrode 3 part through the protecting film 4 so that the analog detection of the A/F in the rich region in facilitated by measurement of Ip.
As shown in FIG. 12 (B), when an electromotive force e.lambda. between two electrodes is detected with the potential of the electrode 3 contacting the exhaust gas through the protecting film as reference, because the value of e.lambda. changes incrementally by approximately i V at the theoretical F/N, the approximate digital detection of .lambda.=1 is facilitated by measurement of e.lambda.. This principle is known from, for instance, Japanese Patent Laid-Open No. 37599/1972.
As shown in FIG. 12 (C), when an exciting voltage of approximately 0.5 V is applied between two electrodes with the electrode 3 as a cathode, the oxygen ions are pumped from the electrode 3 part to the electrode 1 part and the pumping current Ip is measured by the ammeter 5. As this pumping current Ip corresponds to the quantity of the oxygen diffused into the electrode 3 part through the protecting film, the lean region (.lambda.&gt;1) can be detected from the Ip value. This principle is known from, for instance, Japanese Patent Laid-Open No. 69690/1977.
Examples of the characteristics of the sensors of the prior art shown in FIG. (A)-(C) are shown in FIG. 13. The characteristic in the lean region is shown by a one-dot-chain line, the characteristic in the rich region is shown by a dotted line and the characteristic at the theoretical A/F is shown by a solid line. Thus the detection methods which can detect individual regions are known but the constitution with which the A/F is detected smoothly in the wide range by a single method is not proposed yet.
Note that, as the principle of the sensor shown in FIG. 12 (B) is not based upon the speed of diffusion rule, the rate of the gas diffusion resistance of the protecting film 4 of the sensor of FIG. 12 (B) is formed to be smaller than those of the sensors of FIG. (A) and (C). In general, the thickness of the protecting film 4 in the case of FIG. 12 (B) is formed thinner than those in other cases.
It is also known from, for instance, Japanese Patent Laid-Open No. 62349/1980 and Japanese Patent Laid-Open No. 154450/1980 that analog detection of A/F can be obtained from a terminal voltage between two electrodes by applying a certain current between the electrodes and it is also shown that the A/Fs in the rich region and the lean region can be detected by switching the polarities of two electrodes. However, it is not shown when and how the polarities must be switched.
It is also known from Japanese Patent Laid-Open No. 48749/1983 that the theoretical A/F and the A/F in the lean region can be detected by switching the connection between two electrodes and an electronic circuit and changing the measurement mode of the sensor. However, detection in the rich region is not considered in this method.